This invention relates to high resolution and high speed scanning probe microscopes by which material information of a sample surface can be obtained.
The Atomic Force Microscope (AFM) which can measure at the atomic level the surface contour or shape of insulating materials was invented by G. Binnig, C. F. Quate et al, in 1986. It was disclosed by Y. Martin et al (Appl. Phys. Letters, 52, 1103, 1988) that by way of the application of AFM, P-N junction surface potential was measured with high resolution at the sub-nanometer level. And recently, according to H. Yokoyama et al (Mal, Electronics Bio electronics 3, 79 1992) the potential difference of an aluminum-platinum junction was measured. In FIG. 2 the configuration of equipment used by yokoyama is shown.
In the setting where direct potential (V.sub.Bias) for sample potential adjustment is supplied by a direct current bias power supply 5 and alternating current voltage (V.sub.AC sin.omega.t) for probe drive is supplied by an alternating current power system 4 between a sample 1 and a sample stage 2, forced energizing of a probe 3 occurs due to capacitive coupling when probe 3 is very closely set in the neighborhood of a sample.
By detecting the amplitude of this forced energizing by way of a displacement detection system like optical lever system 6, the capacitance gradient between sample and probe is obtained by detecting the 2.omega. element of excitation frequency by lock-in amplifier 7, and the surface potential Vs directly under probe 3 is obtained by detecting the .omega. element of excitation frequency by lock-in amplifier 8.
In order to have a graphic image of both capacitance gradient and surface potential, aiming and keeping that capacitance gradient ##EQU1## should be constant by controlling the distance between probe 3 and sample 1 by using a z-axis servo system 9 and a piezoelectronic scanner 10, distribution of surface potential at this moment is obtained from time to time. As regards methods for obtaining surface potential calibration, a method by which calibration is made by charging a known potential level to direct current bias power supply 5 for sample potential adjustment, or a feedback method by which the direct current bias power level 5 for sample potential adjustment is feedback controlled so that the .omega. element of excitation frequency should be zero, may be applied.
In prior art equipment in which a probe is energized by a 5-10 kHz alternating current voltage, its displacement signal is detected using a Lock-in amplifier. Accordingly in order to obtain a sufficient S-N ratio in relation to the excision frequency, the time constant of the Lock-in amplifier should be kept at 10-20 msec or longer. Even in this situation, 7-14 minutes are necessary for generating a 200-200 element picture. When feedback control of the direct current power for sample potential adjustment is applied, a longer time, including this feed-back control time, is needed for the image acquisition. Even more, only surface potential difference distributions can be obtained by the prior art technology.